This study will investigate the responses of spinal interneurons to physiologically-induced muscle afferent excitation. Extracellular recordings of interneurons will be made within the intermediate nucleus of the lumbo-sacral spinal cord, in anesthetized or decerebrate cats. Muscle afferents from hind-limb muscles will be excited by longitudinal tendon vibration, slow muscle stretch and muscle contraction -contraction will be induced by distributed stimulation of sectioned, subdivided ventral roots. Since vibration is a selective stimulus to primary spindle receptors, interneurons responding specifically to this stimulus may be classed as Ia interneurons. Interneurons responding selectively to muscle contraction, a specific stimulus to tendon organs, will be classed as Ib interneurons. Finally, interneurons responding to slow stretch alone may be classed as secondary interneurons (a group II designation may be too broad). In sum, the use of vibration, slow stretch and muscle contraction may assist a functional classification of muscle-afferent related interneurons. Input-output properties of identified interneurons will be studied using controlled variations in muscle force or length. The synaptic connections of a sample of interneurons will be investigated using combined intracellular recordings from motoneurons and extracellular recordings from interneurons; the extracellular record will drive a spike triggered average of intracellular potential, revealing any correlated synaptic potentials. Connections to other interneurons of the same or different classes will also be explored, using dual extracellular recordings, in an attempt to define fields of local interneuronal interaction. These various interneuronal properties will be used to evaluate interneuronal contributions to the regulation of muscle action in general, and to the decerebrate stretch reflex in particular.